ZOOKEEPER-4785: Txn loss due to race condition in Learner.syncWithLeader() during DIFF sync #1918

Job	Run time
mvn (full-build-jdk8, 8, -Pfull-build apache-rat:check verify -DskipTests spotbugs:check checksty...	2m 23s
mvn (full-build-jdk8, 8, -Pfull-build apache-rat:check verify -DskipTests spotbugs:check checksty...	2m 23s
mvn (full-build-jdk11, 11, -Pfull-build apache-rat:check verify -DskipTests spotbugs:check checks...	2m 34s
mvn (full-build-jdk11, 11, -Pfull-build apache-rat:check verify -DskipTests spotbugs:check checks...	2m 34s
mvn (full-build-java-tests, 11, -Pfull-build verify -Dsurefire-forkcount=1 -DskipCppUnit -Dsurefi...	1h 5m 7s
mvn (full-build-java-tests, 11, -Pfull-build verify -Dsurefire-forkcount=1 -DskipCppUnit -Dsurefi...	1h 5m 7s
mvn (full-build-cppunit-tests, 11, -Pfull-build verify -Dtest=_ -DfailIfNoTests=false)	6h 0m 14s
mvn (full-build-cppunit-tests, 11, -Pfull-build verify -Dtest=_ -DfailIfNoTests=false)	6h 0m 15s
	14h 20m 37s

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